Bonus if you provide an "argument" to back up your point.
My guess is Boram Darkwill because Swain's lore only said he "crushed" Darkwill. It never explicitly says Darkwill died.
All right, folks, your turn!
Duh - it's the old man from the Bard: Mountain teaser video at 1:42. He even has a speaking line!
Come on, people. Do I have to give you everything?!
;)
swain's missing arm
Its Helmet Bro.
It's Star Guardian Urgot, the meme that refuses to die
Whoever it is it clearly is not Yone. I mean could you IMAGINE Riot making yet ANOTHER Ionian champ right after Sett? Crazy right? It has got to be someone else that is way cooler and clearly not just some edge boy from Ionia to milk the playerbase. I mean just look at all the new inovative champs Riot has given us since Zoe! They are all so unique and complex, and clearly not designed to be all either e girl sex appeals, edgy villains tm, or magical lolis (or cats). Anyways I can't wait for the Ruined King. He refuses to die due to the Black Mist and it's heavly hinted he controls it. With the new RK game it's even more possible.
The hero Poppy's been looking for. Obv.
Cithria because it's a familiar face. And damn, LOOK at her face. Cutie. https://cdn.discordapp.com/attachments/365550473429909504/652620746123640840/Cithria_300_fixed.png
Pre-rework Soraka because that thing is a fucking demon from the past that refuses to die
Dude, no.
It's probably Quinn's brother. He's probably someone who refuses to die and now for some reason, blames Quinn for his death.
You know how Riot is with their sibling rivalry fetish.
It's DuCouteau. I also think he and the "masked" edgy solo laner are one in the same.
He went missing, I think most of us can agree it's likely because he posed enough of a threat to the Black Rose that they attempted to eliminate him. What if he didn't get killed or captured, but somehow faked his death/capture to go off the grid?
- "Do they know who killed your father, or do they still blame me?"
Him and Swain are allies. So this voiceline doesn't fit. It's really suspect whether Swain killed him at all.
Grand General Boram Darkwill had been deposed by Jericho Swain, and a number of noble houses had chosen to honor this coup… including Du Couteau. Outraged and disgusted by her husband’s betrayal, and fearing that all members of the Black Rose were now in jeopardy, Soreana became desperate.
- Cassiopiea Bio
When DuCouteau went against The Black Rose he became a target. His wife was DESPERATE and perceived his actions as a BETRAYAL. His life was in danger so when Swain went about purging his enemies and the rest of the Darkwill loyalists he made it seem as though DuCouteau, someone who in truth supported Swain was included among them. That was when Marcus donned the mask and began working within the shadows to dismantle The Black Rose and destroy the corrupting influence that tore apart his family and Noxus because like Swain he is a nationalist at heart.
The way Reav3 describes gives me a really awesome mental image of him as "Noxian Batman".
Even before today I was sure he was coming soon with The Black Rose becoming more fleshed out after Mordekaisers' VGU, the fact that we haven't had a new Noxian champ since 2016, The Lost General Card in Legends of Runeterra that seems to exist purely to tease us and confirm that he still is a relevant thing in the lore.
Now with these subtle clues and the message in the video about wanting to bring more lore champions like Kai'Sa and Senna into the game I will eat my hat if champion 150 isn't him. It would be also fitting to mark champion 150 by making him a lore champion, a champion that has existed as an idea since October 23rd, 2009 in Katarinas' first rendition of the lore, back when she had blue hair and green clothes. That's how you celebrate 10 years of LoL.
My vote is Saijax Cail-Rynx Kohari Icath’un from the short story Where Icathia Once Stood. Obviously, he's an interesting character, as shown by his interactions with the protagonist. He is also from the past, and the Void is kind of like a demonic force, duh. Finally, he has a legitimate reason to return; he was last seen riding off into the sunset after the cataclysmic battle between Icathia and Shurima.
It's a shame that we definitely haven't heard of this guy in any other lore material or in-game material, especially because I think he'd be a really cool Skirmisher champion. Maybe he could even have a unique dodge ability!
Barn the Sentient Barn. Everyone knows Barn. Everyone loves Barn.
The Ruined King!!!
C'mon man, don't you people pay attention to patterns? It's just another Ryze rework, chill.
I believe the new champion will be the Maid from 
The Man with the Steel Cane.
Jhin said he'd reconstruct her face, which is super generous of him!
Faker.
Guys, this is child's play. It's obviously Old Mordekaiser.
We were told he was so buggy and they had to remove him and give us a Noxian champion that resembled him. But it was all a farce and an elaborate play to to truly reveal that Old Mordekaiser was the Ruined King all along!
It makes total sense! They are both entirely different characters where one wants to make people into chairs while the other actually wants to use the souls of those he kills!
Come on guys!
River Shen
Because there is Shen, and then there is River Shen.
Helmet Bro!
It's clearly that yordle that yuumi and book saved.
He got recaptured, but this time Yuumi couldn't help him. He had to do it himself...
Ao Shin
The dude who Noc fuckin killed in that story because Riot can bring him back as an edgy dude and that's cool.
Rasho, the Marai that went to get the pearl before Nami, he's like a spectral void marai who rabbits on about how good the void is.
Sterak. Since people with his item refuse to die. It's time for the item's namesake to appear in game. 
It’s Jhin sthupids
parents who were killed off in the overhaul of his lore! Since we all know that the lore can completly change at any time its just logical that Riot brings them back together with a new overhaul for Zac's lore.
The ghost of all of the eggs Jax has consumed
Yone being brought back from the undead to have his revenge against Yasuo. I mean, Kai'Sa reappeared, Senna was brought back to life, wouldn't be surprised if Yasuo's, most popular champion in the game, his older brother came back as well.
Besides, wouldn't people enjoy beating the crap out of Yasuo with his own bro cause karma? : ^)
If we are being 100% serious, its a poro. What else could it be? Riot is setting up how powerful poros are with legends of runeterra. Sinister poro is unaffected by the black mist! Mighty poro can wield a hammer! Nimble poro is a martial arts master! Whimsical new jungler? Yeah its a poro champ
Helmet Bro The narrator in the audio tale for Pyke The guy ekko fought in his cinematic Neekos frog buddy Carmen Santiago
Who would win?
v 
C.
I wish I wouldn't have to post this here in the "obviously wrong answers" thread, but well.
Roe from the story "Child of Zaun".
She actually didn't die in Urgot's trial, Vi just thought she did. But the gas had a devastating effect on her body and she had to heavily augment herself with chemtech to survive. No longer the innocent girl she has been, she is now forming another crew of Zaunites to fight the Piltovan oppressors. She has a mask, maybe because of her chemtech augmentation, just like Urgot has a mask or because the trial has disfigured her.
She refuses to die, clinging to life with her chemtech augmentations. Would love to see her resembling some kind of insect with her augmentations, like Husk who was heavily inspired by wasps in design. Maybe the group of Zaunites she is leading now could all have insectoid chemtech augmentations, as a hommage to her idol Urgot, the dreadnought, who has a cult using a spider emblem.
It's gonna be Liandry, clearly, and we're finally gonna see her unleash her torment upon the rift.
Zed's shadow clone.
Irelia's little brother.
Corpse of the queen of the blessed isle's
because kalista needs better friends.
Jax's lifelong partner, Laamp, joins the fight!
Someone who can finally fix this game by banning trolls.
